AC/DC or DC/AC converter stations include equipment for converting between high voltage DC and AC currents and are used for electric power transmission, where exemplary DC voltages can be on the order of several 100 kV. The desire arises to measure the DC currents in such converter stations.
Known direct current measurement in high-voltage direct current (HVDC) electric power transmission systems utilizes DC Current Transformers (DCCT) based on automatic DC ampere-turn balancing of primary and secondary currents through zero-flux detection in a transformer magnetic core (Ref. 11). Electrical insulation of DCCTs by paper and oil from high-voltage potential can result in bulky equipment mounted on porcelain insulators.
Optical DCCTs utilising optical fibres for electrical insulation can eliminate the risk of flashover, explosion and environmental hazards due to the application of oil-filled porcelain insulators.
State of the art in HVDC measurement are optical DCCTs based on low-ohmic resistive current sensors included in the primary current circuit. An optoelectronic module placed together with the current shunt at high-voltage potential samples and converts the measured current-dependent resistive voltage drop into a serial data stream. The serial data are transmitted as an optical digital signal via the optical fibre link to the interface in the control room. Power to supply the optoelectronic module can be simultaneously transmitted as laser light from the interface to the current transducer.
The cooling capability of the current transducer housing can limit the thermal current rating of known optical DCCTs for a given design of the current sensor.
The design of fiber-optic current sensors is almost independent of the thermal currents specified for measuring apparatus for HVDC bulk power transmission.
Ref. 1 discloses a concept for stress-free packaging and orientation of the sensing fiber of a fiber-optic current sensor (e.g., for precise measurement of high direct currents at aluminum smelters).